Various mechanical stresses are put on a photographic material coated with a silver halide emulsion, in general. For example, a negative film for general photographing is rolled up into a magazine, folded when loading in a camera, or pulled for frame sliding. Further, an emulsion face is pressed in a swollen state, in some cases, depending on processors when an exposed negative film is development processed.
As described above, when various stresses are put on a photographic material, stresses are put on silver halide grains through gelatin, which is a binder of silver halide grains, and a plastic film support. It is known that if stress is put on silver halide, photographic properties of a photographic material are fluctuated. Examples thereof are disclosed in detail, for example, in K. B. Mother, J. Opt. Soc. Am., 38 (1948), p. 1054, P. Faelens and P. de Smet, Sci. et Ind. Phot., 25, No. 5 (1954), p. 178, and P. Faelens, J. Phot. Sci., 2 (1954), p. 103.
With respect to tabular silver halide grains, production methods and techniques for using thereof have been disclosed in U.S. Pat. Nos. 4,433,048 and 4,434,226, etc. It has been known in this field of industry that the shape of tabular grains has various advantages which contribute to the improvement of sensitivity/graininess relationship, the improvement of sharpness due to the peculiar optical nature of tabular grains and the improvement of covering power, and tabular grains have supported the rapid progress of silver halide photographic materials in recent years.
Because of their peculiar "tabular shape", on the other hand, the performance degradation of photographic properties by stress (pressureability) of tabular grains is large, and therefore, various means have been contrived to cope with this drawback.
For example, U.S. Pat. No. 4,806,461, JP-A-63-220238 and JP-A-3-189642 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") disclose techniques for improving sensitivity/graininess relationship, dependency on illumination intensity of exposure, pressureability and storage stability by introducing dislocation lines into tabular silver halide grains while controlling.
The technique of improving sensitivity/graininess ratio by forming tabular silver iodobromide grains having iodide nonuniformly dispersed in the grains is disclosed in U.S. Pat. No. 4,414,310. Further, JP-A-3-136032, JP-A-3-136033 and U.S. Pat. No. 5,061,616 disclose techniques of improving the desensitization due to pressure by forming a silver bromoiodide thin layer shell which comprise adding iodide to a tabular host emulsion and then prescribing the pAg and the temperature. However, the techniques disclosed therein only refer to the improvement of the desensitization due to pressure among the degradations of photographic properties caused by various stresses. Therefore, the effects of these techniques have been insufficient concerning very important and annoying performance degradations of photographic properties due to other stresses in photographic materials, that is, stress marks by folding and stress marks in a swollen state of a coated film.
While, in recent years, demands for tabular silver halide emulsions have become increasingly strict, in particular, the development of high sensitivity emulsions which are improved in performance degradation of photographic properties due to various stresses has been desired.